High-speed optical soliton transmission using bit rates of decades of gigabits per second or more needs ultrashort optical pulses shorter than 10 ps. Optical pulses having a pulse width of 10 ps or more can relatively easily be generated by a method of driving a semiconductor laser with short pulses or by a method using an electroabsorbtion modulator as an optical gate to transmit a continuous wave light for a short time. With these methods, however, it is difficult to obtain ultrashort optical pulses in the order of several picoseconds required for high-speed optical soliton transmission. As means for compressing the pulse width of optical pulses having an ordinary pulse width, a Dispersion Decreasing Fiber is well known(for example, Morita et al., "Pre-emphasis Optical Soliton Compression Using DDF", The Institute of Electronics, Information and Communication, Digest for the Communications Society Symposium 1995, p. 398).
Dispersion decreasing fibers are optical fibers whose chromatic dispersion values decrease with transmission distance. Such optical fibers exhibit, for example, chromatic dispersion of 12 ps/nm/km to 14 ps/km/nm at their input ends but 1 ps/km/nm to 2 ps/nm/km at their output ends in the wavelength band of 1,500 nm which is regarded useful for long-distance optical transmission.
Japanese Patent Laid-Open No. 6(1994)-281896 discloses an optical pulse generator for generating ultrashort optical pulses usable for optical soliton transmission. This generator uses serially connected two electroabsorbtion modulators that are driven in phases different by 180 degrees to generate ultrashort pulses around 2 ps. This prior art, however, involves difficulties in practical use, such as wide spectral width.
The peak power and the soliton distance required for optical soliton transmission are theoretically given by the following equations. The soliton distance pertains to a transmission distance for characterizing solitons.
The peak power Psol (fundamental soliton) is: EQU Psol=0.776(.lambda.3 A.sub.eff D)/(n.sup.2 cn.sub.2 t.sup.2)(Eq.1)
where .lambda. is the signal wavelength (m), A.sub.eff is the effective core area (m.sup.2), D is the chromatic dispersion value (s/m.sup.2), c is the speed of light (3.0*10.sup.8 m/s), n.sub.2 is the nonlinear refractive index (m.sup.2 /W), and t is the pulse width (s).
The soliton distance z.sub.0 is: EQU z.sub.0 =0.322(n.sup.2 c/.lambda..sup.2)(t.sup.2 /D) (Eq.2)
where .lambda. is the signal wavelength (m), D is the chromatic dispersion value (s/m.sup.2), c is the speed of light (3.0*10.sup.8 m/s), and t is the pulse width (s).
In case of optical soliton pulses, since the product of the energy E and the pulse width t is proportional to the chromatic dispersion D, the pulse width t can be decreased by decreasing the chromatic dispersion D amply slowly (adiabatically) in the propagating direction by a ratio larger than the decrease in energy due to the propagation loss. Dispersion decreasing fibers use this theory. Note here that the pulse width compression ratio is equal to the decreasing ratio of the chromatic dispersion D under an ideal condition with no propagation loss; however, if any optical loss a exists, then the pulse width compression ratio becomes a value obtained by multiplying the chromatic dispersion decreasing ratio by exp(aL) where L is the fiber length.
The conventional device combining a pulse-driven semiconductor laser and a dispersion decreasing fiber is subject to variations in laser oscillating frequency due to the pulsative activation of the laser, which results in broadening the spectrum width of the optical pulse too much. To cope with the problem, the conventional device uses a narrow band optical filter to remove undesired frequency components. This approach, however, involves the disadvantage that pulse widths are limited by the band width of the narrow band optical filter. Additionally, it is almost impossible to fully remove varieties in oscillating frequency due to direct modulation of the semiconductor laser.
In devices combining an electroabsorbtion modulator and a dispersion decreasing fiber, for example, optical pulses of 14.6 ps produced by the electroabsorbtion modulator can be compressed to 8 ps by introducing them to a 15 km-long dispersion decreasing fiber whose dispersion decreases from 13.7 ps/nm/km to 2.3 ps/nm/km. For this purpose, however, the input power to the dispersion decreasing fiber is up to or beyond a value satisfying the soliton condition. That is, since the pulse width of optical pulses introduced to the dispersion decreasing fiber is relatively wide, as wide as 14.6 ps, the soliton length becomes long, and the length of the dispersion decreasing fiber also becomes long, as understood from Eq. 2. As a result, the propagation loss increases. In order to compensate the propagation loss, the conventional device increases the peak power of input optical pulses to the dispersion decreasing fiber beyond the theoretical value obtained from Eq. 1, and hence results in a mode of use which does not meet the soliton condition.
Unless the soliton condition is satisfied, the time waveform and the spectral shape of optical pulses are distorted unacceptably for use in optical soliton transmission. In contrast, if the soliton condition is satisfied, then the pulse width compression effect by the dispersion decreasing fiber is restricted.